THE GLY1 GENE OF SACCHAROMYCES-CEREVISIAE ENCODES A LOW-SPECIFIC L-THREONINE ALDOLASE THAT CATALYZES CLEAVAGE OF L-ALLO-THREONINE AND L-THREONINE TO GLYCINE - EXPRESSION OF THE GENE IN ESCHERICHIA-COLI AND PURIFICATION AND CHARACTERIZATION OF THE ENZYME

The GLY1 gene of Saccharomyces cerevisiae is required for the biosynth esis of glycine for cell growth [McNeil, J. B., McIntosh, E. V., Taylo r, B. V., Zhang, F.-R., Tang, S. & Bognar, A. L. (1994) J. Biol. Chem. 269, 9155-9165], but its gene product has not been identified. We hav e found that the GLY1 protein is similar in primary structure to L-all o-threonine aldolase of Aeromonas jandiae DK-39, which stereospecifica lly catalyzes the interconversion of L-allo-threonine and glycine. The GLY1 gene was amplified by PCR, with a designed ribosome-binding site , cloned into pUC118, and expressed in Escherichia coli cells. The enz yme was purified to homogeneity, as judged by polyacrylamide gel elect rophoresis. The enzyme has a molecular mass of about 170 kDa and consi sts of four subunits identical in molecular mass. The enzyme contains 2 mol pyridoxal 5'-phosphate/4 mol of subunit as a cofactor, and its a bsorption spectrum exhibits maxims at 280 nm and 420 nm. The enzyme ca talyzes the cleavage of not only L-allo-threonine to glycine but also L-threonine. We have termed the enzyme a low-specific L-threonine aldo lase to distinguish it from L-allo-threonine aldolase.